A. Field of the Invention
The present invention relates to apparatuses for detection of radiation. More specifically, one embodiment of the present invention relates to a multichannel nanoparticle scintillation microdevice with integrated waveguides that is capable of detecting and discerning multiple types of penetrating radiation when used in combination with an optical light detector. Another embodiment of the present invention also relates to scintillating materials.
B. Description of Related Art
Fissionable weapons grade material emits a variety of ionizing radiation, including alpha, beta, gamma, X-ray and neutron radiation. Therefore, homeland security applications require a portable and/or clandestine field detector that is capable of detecting and discerning the composition of concealed nuclear material of many types and constituencies, including that nuclear material used in weapons. The detection of neutrons, gamma rays, and X-rays by such a portable and/or clandestine field detector is of significant utility, as those particles cannot be shielded as easily as alpha and beta particles.
Neutrons are commonly detected by BF3 tubes, but these devices are relatively large, with approximate sizes in the tens of centimeters, and efficiency of BF3 tubes is proportional to the physical dimensions of the tube (Westinghouse Nuclear Instruments Catalog (author unknown) 1959). In these tubes, neutrons interacting with 10B release an alpha particle. The charged alpha particle ionizes the fill gas liberating free electrons that are attracted to the anode, producing a current. Apart from the bulky shape of the device, the gases inside are extremely toxic, and the energy of the incident radiation cannot be determined.
Gamma and X-rays, on the other hand, are commonly measured by gas electron multiplier (GEM) detectors (Bambynek, W. “On Selected Problems in the Field of Proportional Counters,” Nuclear Instruments and Methods, 112, 103-110 (1973)). Within a GEM detector, a gamma ray ionizes gas particles inside of a housing, thus releasing electrons. An applied high electric field between a anode and cathode causes the electrons to increase in energy, which, in turn, increases electron velocity. Enough energy is supplied by this increase in velocity to ionize more gas atoms, which creates an avalanche cascade of electrons (Va'ra, J. and T. Sumiyoshi, “Single electron amplification in a ‘single MCP+micromegas+PADS’ detector,” IEEE Transactions on Nuclear Science, (51) 5, 2262-2266 (2004)). Moreover, a voltage ranging from 500 to 3000 V is required to produce the high electric field and GEM detectors are only used to detect photon-based radiation (Chechik, R.; Breskin, A.; Guedes, G. P.; Mormann, D.; Maia, J. M.; Dangendorf, V.; Vartsky, D.; Dos Santos, J. M. F.; Veloso, J. F. C. A., “Recent Investigations of Cascaded GEM and MHSP Detectors,” IEEE Transactions on Nuclear Science, (51) 5, 2097-2103 (2004)).
The Geiger counter is another traditional portable radiation detection instrument. Geiger counters and similar detectors contain a gas-filled tube that conducts electricity when a particle or photon of radiation briefly causes the gas to be conductive. The detection instrument amplifies this signal and displays it to the user, either as a current measurement or an audible click. Geiger counters and the like measure, by counting incidents only, alpha, beta, gamma and X-ray radiation. Accordingly, no analysis of the composition of a concealed material containing multiple radiation types is possible and neutrons are not counted, in any event. Moreover, the resulting device is large and bulky. A very small Micro Geiger counter has been reported, but this device measures only beta particles (Wilson, C.; Eun, C.; Gianchandani, Y., “D-MicroGeiger: A Microfabricated Beta-Particle Detector with Dual Cavities for Energy Spectroscopy,” IEEE MEMS 2005, 622-625 (2005)).
Another popular detection instrument, the scintillation counter, uses a scintillator to convert beta particles (known as fast electrons) into optical pulses (Price, W., Nuclear Radiation Detection, 2d ed., Mc-Graw-Hill, New York (1964)). The scintillator consists of a transparent crystal, plastic, or an organic liquid that fluoresces when struck by ionizing radiation. An optical light detector, e.g., a photomultiplier tube or photodiode, then measures the light emanating from the scintillator by converting it to an electric current (Birks, J. B., The Theory and Practice of Scintillation Counting, Pergamon Press, Oxford, UK (1964)). Some scintillators have been adapted to measure alpha particles and others measure gamma particles, converting these species into pulses of light (Knoll, G. K., Radiation Detection and Measurement, 3d ed., John Wiley & Sons, Inc., Danvers, Mass. (2000); Petr, I.; Birks, J. B.; Adams, A. “The Composite directional gamma-ray scintillation detector,” Nuclear Instruments and Methods, (92) 2, 285-293 (1972); Viday, Y. T.; Grinyov, B. V.; Zagarij, L. B.; Zverev, N. D.; Chernikov, V. V.; Tarasov, V. A.; Kudin, A. M., “Research and development of ceramic scintillators applied to alpha-particle detection,” IEEE Nuclear Science Symposium & Medical Imaging Conference, 2, 762-63 (1995)). Article counting and quantification of the amplitude of the signal produced by the optical detector typically can be accomplished. Scintillation counters are widely used because they can attain good quantum efficiency despite being moderately inexpensive.
The most widely used scintillators include inorganic crystals, organic-based liquids and plastics. Inorganic scintillators have the highest light output and linearity, but tend to have relative slow response times. Organic scintillators are much faster, but have lower light output. Moreover, the high Z-value and high density of inorganic crystals are suited for gamma/X-ray spectroscopy, whereas organics tend to be better suited for the direct detection of beta and alpha particles. Plastic scintillators, solids consisting of a polymerized organic scintillator in solution, are useful due to easy shaping and fabrication, and come in a variety of standard sizes and shapes. Additionally, plastics are relatively inexpensive as compared to inorganic crystals. It is desirable to use plastic scintillators in small radiation detection devices like those in certain embodiments of the present invention because such plastics are easily extruded into a variety of shapes and sizes to be used in microdevices, yet remain substantially solid.
Because organic scintillators, including plastics, demonstrate no photopeak due to a low Z-value, those in the art have loaded such scintillators with high-Z elements to provide for some possibility of photoelectric conversion upon exposure to gamma/X-rays. The addition of such elements, however, may lead to lower light output and the achieved energy resolution may be inferior to that of inorganic crystal scintillators. It is thus desirable to achieve particle loading that remains transparent, uniformly thin, and easily patterned onto microdevices.
Further, transparent fast-electron plastic scintillating materials specifically tailored to provide optimized detection of neutrons, in addition to alpha particles, beta particles, and gamma and X-rays, are useful in microfabricated devices, particularly those used to detect fissionable weapons grade nuclear material. Gadolinium has the highest thermal neutron capture cross-section of any known element (255,000 barns) and is thus ideal for thermal neutron capture reactions. Previous efforts to utilize gadolinium to detect neutrons involved its use as a foil (see, e.g., U.S. Pat. No. 5,659,177) or macroscopic block (see, e.g., U.S. Pub. No. 2005/0105665; U.S. Pub. No. 2005/0127300, U.S. Pat. No. 5,057,692).
As will be further described below, doping an organic plastic scintillator with various charge conversion nanoparticles to create transparent novel scintillating materials allows for the microfabrication of devices that are capable of converting differing radiation species into electrons through independent physical mechanisms (see Pellegrin, S., Whitney, C., Wilson, C. “A Multichannel Nanoparticle Scintillation Device With Integrated Waveguides for Alpha, Beta, Gamma, X-ray and Neutron Detection,” Proceedings, IEEE MEMS (2006), which is hereby incorporated in its entirety herein). Due to the flexible nature of the scintillator base, each tailored scintillator may be manufactured into a thin film and integrated into a translucent or transparent substrate to create a novel microdevice capable of detecting and discriminating all species of radiation emitting from fissionable weapons materials in an expedient and efficient manner, and at relatively low cost.